1. Field of the Invention
The present invention relates to a method for manufacturing a rhenium-containing alloy powder whose main component is nickel or a metal that can be alloyed with rhenium, such as platinum, palladium, iron, cobalt, ruthenium, or rhodium, and more particularly relates to a method for manufacturing a rhenium-containing alloy powder that can be used suitably in a conductor paste used to form internal conductors in laminated ceramic electronic parts.
2. Description of the Related Art
In the field of electronics, conductor pastes, resistor pastes, and other such thick film pastes are used to manufacture parts such as IC packages, capacitors, resistors, electronic circuits, etc. These pastes are produced by uniformly mixing and dispersing conductive particles of a metal, an alloy, a metal oxide, or the like in an organic vehicle along with a vitreous binder or any other additives that are needed, and the resulting pastes are applied to substrates, and then firing at a high temperature to form conductors or resistors.
Laminated ceramic electronic components, such as laminated capacitors and laminated inductors, or ceramic multilayer substrates are generally manufactured by alternately laminating an unfired (green) ceramic sheet of a dielectric, a magnetic material, or the like, and an internal conductor paste layer, in a plurality of layers of each, and firing all the layers at the same time at a high temperature. It used to be that palladium, silver-palladium, platinum, and other such noble metals were most often used as the internal conductor, but more recently the use of nickel and other such base metal materials has become increasingly popular because of the need to conserve resources and to reduce the delamination and cracking caused by oxidation expansion during the firing of palladium or silver-palladium, and so forth.
There is a trend toward increasing the number of laminations with these laminated parts and multilayer substrates, to the point that laminated capacitors, for instance, are beginning to be manufactured with hundreds of layers of lamination. This has made it necessary to reduce the film thickness of the ceramic layers, and in turn to further reduce the film thickness of the internal conductor layer. For example, if the thickness of a ceramic layer is about 3 μm, unless the internal conductor film thickness is 1 μm or less, and preferably about 0.5 μm, the middle part of the laminate will end up being too thick, and this can lead to structural defects and diminished reliability.
However, when ordinary nickel particles are used for an internal conductor paste, excessive sintering of the nickel particles during firing can cause them to clump together or cause abnormal particle growth, so not only does the internal conductor become a discontinuous film, which can lead to higher resistance, or to circuit disconnection, but another problem is that the conductor becomes thicker, so there has been a limit to how thin a film could be made. Specifically, when nickel particles are fired in a non-oxidizing atmosphere, such as an inert atmosphere or a reducing atmosphere, in order to prevent oxidation, their sintering begins early, and even single crystal particles with relatively low activity begin to sinter and shrink at a low temperature of 400° C. or lower.
Meanwhile, the temperature at which a ceramic layer starts to sinter is generally much higher than this. For example, the temperature is approximately 1200° C. with barium titanate, and when a ceramic green sheet of this and a nickel internal conductor paste layer are alternately laminated in a plurality of layers of each, and all these layers are fired at the same time at a high temperature, the ceramic layers do not shrink together with the nickel films, so the nickel films are pulled in the planar direction. Consequently, it is presumed that small voids produced in the nickel films by sintering at a relatively low temperature expand into large holes as the sintering proceeds at higher temperatures, or that this is accompanied by growth of the film in the thickness direction.
Therefore, to reduce the thickness of the nickel internal conductor layers, it seems to be necessary to make the nickel particles finer and give them better dispersibility, so that as few voids as possible are created during firing, and to match the sintering shrinkage behavior with that of the ceramic layers. Also, even when the films are formed thicker, this mismatching of the sintering shrinkage behavior between the conductor layers and the ceramic layers causes delamination or cracking and other such structural defects, and is therefore a problem in that it lowers the yield and the reliability of the product.
Various attempts have been made in the past to suppress the sintering of conductor layers up to the sintering commencement temperature of the ceramic layers. For example, the sintering commencement of conductor layers can be apparently delayed to about 800° C. by adding ceramic particles with the same composition as that used in the ceramic layer to the conductor paste. However, since the sintering of the metal particles themselves in the conductor layer is not being suppressed, when the material is fired at a high temperature of about 1300° C., the conductor layer still loses its continuity and conductivity. Also, there is no effect unless these additives are used in a large quantity, so other problems such as higher resistance, etc., arise.
Patent Document 1, listed below, states that the sintering commencement temperature of a conductor paste can be raised by using an alloy powder composed of nickel and at least one element selected from among vanadium, chromium, zirconium, niobium, molybdenum, tantalum, and tungsten as the metal powder used for the conductor paste used in forming the internal conductor of a laminated ceramic capacitor. Nevertheless, the elements disclosed in Patent Document 1 are all baser metals than nickel, so even when the firing is performed under conditions under which nickel will not be oxidized, these other metals often ended up being selectively oxidized. As a result, there is the danger that they will react with the surrounding ceramic and adversely affect the electric characteristics of the laminated ceramic electronic part.
In view of this, various studies have been conducted to find the ideal metal elements for alloying with nickel, and attention has recently been directed to rhenium. Rhenium is one of high-melting point metals, and it is expected to be very effective at suppressing sintering when used for the formation of an internal conductor used in laminated ceramic electronic parts. For instance, Patent Document 2, listed below, discloses a composite powder in which nickel is coated with rhenium.
However, while rhenium is more noble than nickel, it cannot really be considered to have low chemical reactivity, and rhenium oxide in particular sublimates at a low temperature of just a few hundred degrees centigrade. This means that when a rhenium powder or a rhenium-coated metal powder is used to form conductors for electronic parts, the material must be handled with the greatest of care to avoid the oxidation of the rhenium during firing and so on. Alloying nickel and rhenium is thought to be advantageous in terms of suppressing this reactivity of rhenium.
Still, with the alloy powder manufacturing methods known up to now, it was difficult to stably produce alloy powders that were homogeneous and had a small particle size, and alloy powders of nickel and rhenium were particularly difficult to manufacture.
For instance, Patent Document 1 discusses the manufacture of an alloy powder by heating together chlorides of metal elements contained in the alloy powder, evaporating them and mixing these vapors, and then subjecting them to hydrogen reduction, but with a CVD (Chemical Vapor Deposition) method such as this, the particles of the various metal elements typically are not alloyed, and instead are produced individually.
Also, it is possible that PVD (Physical Vapor Deposition) could also be utilized if the vapor pressures of the metals constituting the alloy were close enough to each other, but when the vapor pressures are greatly different, as is the case with nickel and rhenium, it is exceedingly difficult to control the alloying ratio, so a homogeneous nickel-rhenium alloy power cannot be obtained consistently. Because of this, with a powder obtained by a conventional vapor deposition method, the particles of the various metal elements typically are not alloyed, and instead are produced individually, so the product ends up being either a mixed powder in which particles of the various metal elements are both present, or, even if the elements can be successfully alloyed, the powder ends up being one with considerable variance, in which the particle form and average size, the alloying ratio, and so forth are not uniform. When a powder such as this is used to form a conductor for a laminated ceramic electronic part, this lack of uniformity precludes obtaining good electric characteristics.
There is also known a wet reduction method (co-precipitation method) in which aqueous solutions of the metal ions constituting the alloy particles are mixed, and this mixture is then reduced to precipitate a powder, but most of the powder that is precipitated ends up as an agglomeration of fine particles of the various metal elements, and a separate heat treatment is necessary to alloy these agglomerated fine particles. Since the agglomeration further proceeds during this heat treatment, it becomes even more difficult to obtain a fine powder with a uniform particle size. Furthermore, if the surface of the unalloyed agglomerated powder is oxidized into rhenium oxide during heating, since rhenium oxide sublimates even at relatively low temperatures, this process is unsuited to the production of an alloy containing rhenium.
Other known methods include atomization and pulverization, but there is a limit to the size of the powder obtained with either of these, and it has been extremely difficult to obtain a powder with an average particle size on the order of 0.05 to 1.0 μm, which is needed nowadays to form internal conductors for laminated ceramic electronic parts.
Spray pyrolysis is another known method for manufacturing an alloy powder. As discussed in Patent Documents 3, 4, and 5, listed below, and elsewhere, spray pyrolysis is a process in which a solution containing one or more kinds of metal oxide, or a suspension in which these have been dispersed, is sprayed to form fine droplets, these droplets are heated to a temperature higher than the pyrolysis temperature of the metal compounds, and preferably a high temperature that is close to or above the melting point of these metals, and the metal compounds are pyrolyzed, thereby precipitating a metal or alloy powder. This method yields a high-density, highly dispersible, truly spherical metal powder or alloy powder that is either highly crystalline or in the form of single crystals. Unlike a wet reduction process, this method does not require any solid-liquid separation, so manufacture is easier, and since the method involves no additives or solvents that would effect purity, it has the advantage of yielding a high-purity powder containing no impurities. Furthermore, the particle size is easy to control, and the composition of the produced particles basically matches well the composition of the starting metal compounds in the solution, so another advantage is that the composition is easy to control.
However, when a nickel-rhenium alloy powder is manufactured with this method, a solution containing nickel and rhenium is sprayed and pyrolyzed, but because of the above-mentioned characteristics of rhenium, heating causes just the rhenium component to vaporize and separate, so a powder of nickel alone is all that is actually obtained by pyrolysis. This means that a nickel-rhenium alloy powder cannot be obtained by a conventional spray pyrolysis process.
The manufacturing methods discussed in Patent Documents 6 and 7 listed below, are also known. With the methods described in these publications, at least one kind of thermally decomposable metal compound powder is supplied by carrier gas to a reaction vessel, the metal compound powder is dispersed in the gas phase at a concentration of 10 g/L or less, and in this state the powder is heated at a temperature higher than the decomposition temperature and not lower than (Tm−200)° C., where Tm° C. is the melting point of the metal, in order to produce metal powder. This method makes it easy to obtain a metal powder that has spherical particles, good crystallinity, and high dispersibility. It is also possible to obtain a single crystal metal powder by heating the raw material compound powder at a temperature of not lower than the melting point of the metal. Since no additives or solvents that would effect purity are used, a high-purity powder containing no impurities is obtained. Furthermore, a metal powder of uniform particle size can be obtained by controlling the particle size of the raw material powder, so the adjustment of particle size is also easy. There is therefore no need for a classification step, and an extremely fine powder with a narrow particle size distribution that is suited to a thick film paste can be obtained. Also, since the raw material are not put in the form of a solution or suspension, energy loss through evaporation of the solvent is lower than with an ordinary spray pyrolysis method, and the powder can be manufactured more simply and less expensively. Moreover, there is no problem with agglomeration of droplets, and the powder can be dispersed in the gas phase at a relatively high concentration, so efficiency is higher.
Nevertheless, when a nickel-rhenium alloy powder is manufactured with this method, a thermally decomposable metal compound powder containing nickel and rhenium must be prepared as the raw material powder. Chlorides, nitrates, carbonyls and other such compounds with a relatively simple structure, and so forth can be used as thermally decomposable raw material powders, but because these compounds have a low pyrolysis temperature, it is difficult to control their alloying quantitatively. An organic acid salt with a relatively high decomposition temperature, such as a formate, acetate, or oxalate, is thought to be good for improving this control, but when it comes to rhenium, synthesis is extremely difficult, and this complicates manufacture.
As discussed above, with the methods known in the past for manufacturing an alloy powder, if an attempt was made to manufacture an alloy powder containing rhenium, it was difficult to obtain an alloy powder that had a small average particle size, excellent dispersibility, and a uniform alloying ratio.    Patent Document 1: Japanese Patent Publication 2002-60877A    Patent Document 2: Japanese Patent Publication 2004-319435A    Patent Document 3: Japanese Patent Publication 62-1807A    Patent Document 4: Japanese Patent Publication 6-172802A    Patent Document 5: Japanese Patent Publication 7-216417A    Patent Document 6: Japanese Patent Publication 2002-20809A    Patent Document 7: Japanese Patent Publication 2004-99992A